How to find the code coverage of an application for manual testing?

I want to find the code coverage of manual testing and automatic testing (appium) for an Android app (Android studio + gradle).

There are a few questions related to this problem on StackOverflow already but none of them works for me. I have followed the steps of this question: Android Coverage launch with JaCoCo The jacoco.exec file that is being generated in sdcard/ is 37 bytes and is basically empty.

Current configuration of build.gradle:

apply plugin: 'com.android.application'

apply plugin: 'jacoco'
def coverageSourceDirs = [
        '../app/src/main/java'
]

jacoco{
    toolVersion = "0.7.4.201502262128"
}

task jacocoTestReport(type: JacocoReport) {
    group = "Reporting"
    description = "Generate Jacoco coverage reports after running tests."
    reports {
        xml.enabled = true
        html.enabled = true
    }
    classDirectories = fileTree(
            dir: './build/intermediates/classes/debug',
            excludes: ['**/R*.class',
                       '**/*$InjectAdapter.class',
                       '**/*$ModuleAdapter.class',
                       '**/*$ViewInjector*.class'
            ])
    sourceDirectories = files(coverageSourceDirs)
    executionData = files("$buildDir/outputs/code-coverage/connected/coverage.exec")
    doFirst {
        new File("$buildDir/intermediates/classes/").eachFileRecurse { file ->
            if (file.name.contains('$$')) {
                file.renameTo(file.path.replace('$$', '$'))
            }
        }
    }
}
// this is for the report
...

jacoco.java:

package anubhav.calculatorapp;

import android.util.Log;

import java.lang.reflect.Method;

public class jacoco {
    static void generateCoverageReport() {
        String TAG = "jacoco";
        // use reflection to call emma dump coverage method, to avoid
        // always statically compiling against emma jar
        String coverageFilePath = "/sdcard/coverage.exec";
        java.io.File coverageFile = new java.io.File(coverageFilePath);
        try {
            Class<?> emmaRTClass = Class.forName("com.vladium.emma.rt.RT");
            Method dumpCoverageMethod = emmaRTClass.getMethod("dumpCoverageData",
                    coverageFile.getClass(), boolean.class, boolean.class);

            dumpCoverageMethod.invoke(null, coverageFile, false, false);
            Log.e(TAG, "generateCoverageReport: ok");
        } catch (Exception  e) {
            Log.e("jacoco", "inside catch. no emma jar found");
            new Throwable("Is emma jar on classpath?", e);
        }
    }
}